There are known in the art devices for measuring and indicating the penetration of a dental instrument into a tooth, e.g., for root canal treatments. Such devices, called apex locators, identifying the location of the biological apex, are based on relative measurement between a flexible disc-shaped depth stopper, slidingly affixed on the shaft of a dental instrument, leaning against the upper surface of the tooth to be treated, and a predetermined bore length which is believed the tooth can accommodate. It can be easily realized that the use of, and dependency on, the reference marker such as a flexible disc-shaped stopper, leaning against or touching the uneven wavy upper surface of a tooth, is less than desirable when it is necessary to attain accuracies of less than e.g., 0.5 mm. To be on the safe side, dentists take a precautionary distance of say between 0.5 mm and 1 mm from the predetermined apex, thereby leaving the edge of the root canal untreated, eventually constituting a source for development of future infection.